1. Field of the Invention
This invention relates to an air-fuel ratio control system for multi-cylinder internal combustion engines, which controls an amount of fuel to be supplied to the engine by means of feedback control based on an adaptive control theory.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines is known, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 8-232733, in which an adaptive parameter-adjusting mechanism calculates adaptive parameters and an adaptive controller carries out feedback control of the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio by using the adaptive parameters. In the known air-fuel ratio control system, an air-fuel ratio sensor arranged in the exhaust system of the engine detects the air-fuel ratio of the mixture and supplies a signal indicative of the detected air-fuel ratio to the adaptive controller, which in turn determines an amount of fuel to be supplied to the engine such that the detected air-fuel ratio becomes equal to a desired air-fuel ratio.
In the air-fuel ratio control system disclosed in the above publication, values of the adaptive parameters calculated in the past by the adaptive parameter-adjusting mechanism are averaged, and the adaptive parameter values thus averaged are used by the adaptive controller, so as to prevent the adaptive control from being adversely influenced by a particular cylinder when the adaptive control is carried out in a manner synchronous to the combustion cycle of the engine.
Further, if the calculation of a plurality of adaptive parameters is carried out with the same repetition period as a repetition period of calculation of the amount of fuel to be supplied to the engine, e.g. a repetition period of generation of TDC pulses, the amount of calculation increases to increase load on a CPU which calculates the adaptive parameters. To avoid this inconvenience, the repetition period T.theta. of calculation of the adaptive parameters is set longer than the repetition period TF of calculation of the amount of fuel to be supplied to the engine, e.g. to a period equal to TF.times. the number of cylinders, to thereby reduce the amount of calculation of the adaptive parameters to reduce the load on the CPU.
In the conventional air-fuel ratio control system, however, when the averaged adaptive parameter values are calculated by the adaptive parameter-adjusting mechanism with the repetition period T.theta. of calculation set longer than the repetition period TF with which the adaptive controller calculates the amount of fuel supplied to the engine, a plurality of values of each adaptive parameter calculated at the repetition period T.theta. at different times are averaged and the averaged parameter values are used by the adaptive controller. This brings about the following problem:
The adaptive parameter values used by the adaptive controller each contain a component of a particular frequency corresponding to the reciprocal of the repetition period T.theta. and harmonic components of the same, whereas the output from the air-fuel ratio sensor does not contain components corresponding, especially, to the harmonic components due to the low-pass characteristic of the sensor. This increases an identification error in the adaptive parameters, resulting in divergence of the adaptive control.
More specifically, the adaptive parameter-adjusting mechanism is capable of carrying out accurate identification of the adaptive parameters on condition that frequency components of output data (=the adaptive parameters) from the adjusting mechanism are fed back as input data to the adjusting mechanism without being lost after being transformed through a controlled variable (=the amount of fuel to be supplied to the engine), a plant to be controlled (=the multi-cylinder engine), and a plant output sensor (=the air-fuel ratio sensor). However, in actuality the components corresponding to the harmonic components are lost due to the low-pass characteristic of the air-fuel ratio sensor and not fed back to the adaptive parameter-adjusting mechanism. This causes the above-mentioned increase in the identification error.
Further, since the repetition period T.theta. of calculation of the adaptive parameters is set longer than the repetition period TF of calculation of the amount of fuel to be supplied to the engine, the calculated adaptive parameter values cannot be quickly changed in response to changes in operating conditions of the engine without delay, which hinders the amount of fuel to be supplied to the engine from being quickly converged to a desired value. This inconvenience becomes more marked through averaging of adaptive parameters since the averaged values of the adaptive parameters are applied to the calculation of the amount of fuel with the repetition period T.theta..